Field of the Invention
The present invention relates to a radiation curable or photocurable composition for water scavenging layer, to be advantageously used, for example, in a multilayer barrier stack, which may be employed, for instance, in the manufacturing of an organic opto-electric or opto-electronic device such as Organic Light Emitting Diode (OLED).
The present invention further relates to a method of manufacturing the radiation curable composition or photocurable composition, the water scavenging layer, the multilayer barrier stack and the organic opto-electric device or OLED itself.
Related Art
Exposure of moisture sensitive devices such as organic LEDs (both small molecule and polymer based), OPV, CI(G)S solar cells, to the ambient atmosphere results in loss of performance of the device. In the case of OLEDs, ingress of water or of other oxidizing materials can lead to degradation of the active organic layers leading to loss of efficiency mainly due to oxidation of the cathode, leading to local failure of the device. Water ingress can come from two sides, from the anode side or the cathode side. Current state-of-the-art OLEDs are protected from water ingress by using glass as a substrate and glass or metal lids to encapsulate on the cathode side. Conventionally, encapsulation is performed with a coverlid glued at the edges. A getter is used to consume water that might penetrate through the glue. This encapsulation method is expensive and is not functional for large-area devices, especially flexible ones. A more cost-effective alternative, which also will allow flexible devices to be manufactured, is the use of thin film barriers, which can be applied on a plastic foil to act as a substrate and which can be used as a final encapsulation. In order to understand the issues with such kind of barrier, a brief explanation is given below about the mechanism of water ingress in an OLED.
The cathode in an OLED device most often consists of a thin (1-50 nm) layer of Ba (polymer LED) or LiF (small molecule OLED) covered with a relatively thick Al layer. Aluminum would be an excellent barrier against water, if not for the fact that it contains pinholes, of which most of them are caused by particles. Such particles originate from a plurality of causes and their presence is in practice difficult to avoid. Water from the ambient atmosphere is penetrating through pinholes in the cathode layer. Oxidation of metal at the cathode-polymer interface prevents electron injection from the cathode into the polymer during operation of the device, thus introducing a local spot without emission, i.e. a black spot in the bright electroluminescent background. The evolution of the black spots is determined by the diffusion rate of water from the pinhole. The area of the resulting circular shaped spots increases linearly with time. Black spot formation and growth is a shelf effect, i.e. no current or voltage is necessary to drive the process. When an inorganic barrier layer is applied on top of the OLED, the majority of the particles is covered, resulting in a corresponding decrease in the number of black spots. Still the remaining black spot density is by far too large for any practical application. Increase of the thickness of the barrier layer hardly reduces the pinhole density. Once a pinhole is present in such a layer, it tends to propagate while depositing more of the same material.
Graff et al. describe in “Mechanisms of vapor permeation through multilayer barrier films: Lag time versus equilibrium permeation”, J. of Applied Physics, Vol. 96, Nr. 4, pp. 1840-1849 a nowadays common strategy to interrupt the growth of pinholes by a barrier stack with organic layers. In this way, the pinholes in subsequent barrier layers are decoupled resulting in a tortuous path for water transport from the ambient atmosphere to the cathode in the device. Also other layers of different chemical composition, such as different inorganic materials are used for this purpose. Graff et al. investigated the use of polymer decoupling layers having a thickness in the range of 0.1 to 3 μm and suggested that even thinner polymer decoupling layers could result in further improvement.
US2009289549A describes an OLED display provided with a multi-layered protective barrier stack, wherein organic and inorganic layers are alternately stacked in a repeated manner and at least one moisture absorbing layer or water scavenging layer is interposed in the multi-layered protective layer. In particular, US2009289549A describes an embodiment wherein the multilayered protective layer comprises a first inorganic layer, a moisture absorbing layer (water scavenging layer), an organic layer and a second inorganic layer in this order. The presence of the moisture absorbing layer further reduces the ingress of water towards the opto-electric element. The moisture absorbing layer is formed of an organic metal compound solution and may contain additives such as a metal or a metal oxide. The moisture absorbing layer may have a thickness in the range of 3 to 50 nm. It is remarked in US2009289549A that the organic layer between the moisture absorbing layer and the second inorganic layer may have a thickness larger than the thickness of the moisture absorbing layer. The cited US patent does not disclose more specifically how much larger the thickness should be, but the drawing that is referred to suggests that the second organic layer is about two to three times thicker.
The deposition and manufacturing of a suitable moisture absorbing layer or water scavenging layer for opto-electrical devices has revealed exceptional technical difficulties from the point of view of the chemistry and of the required physical properties. No suitable materials have been found up to now, which fulfill all physical, mechanical, optical and processing requirements demanded by the industrial manufacturing of opto-electrical devices and OLED.
Calcium oxide is for example highly hygroscopic, and is useful as a moisture absorbent and a dehydrating agent, in particular in electronic enclosures where moisture decreases the device lifetime. From a viewpoint of handling in opto-electrical applications it would be however necessary to provide calcium oxide in the form of a homogeneous stable liquid, which could be coated or printed and cured on different substrates and at different thicknesses. From a viewpoint of processing in certain opto-electric applications, in particular for making individual layouts and for on demand printing, it would be necessary to be able to deposit the hygroscopic material as a curable ink via inkjet printing, valve jet printing, and liquid dispensing methods. For patterning via gravure printing the viscosity of the ink must be lower than 200 mPa·s at 20° C., and for flexo printing the viscosity of the ink must be lower than 500 mPa·s at 20° C. These are among favoured roll to roll printing techniques. It can be also necessary to have access to medium and low viscosity resins exhibiting the water scavenging properties.